One of the most important issues facing
marine aquarists is providing a suitable environment for their
aquaria's inhabitants. Among the important properties for
a marine environment's suitability is the water's salinity
level. Water that is either too saline or not saline enough
can be stressful or lethal to many organisms. Deciding what
salinity level to maintain in a reef aquarium can be a complicated
task, especially if the organisms come from different environments.
This article includes a brief discussion of how to select
an appropriate salinity target, but selection is not the main
purpose of this article.

Monitoring the salinity level is an important
issue itself, wholly apart from deciding what salinity level
to target in an aquarium. Fortunately, a number of different
methods for monitoring salinity are available to aquarists,
including specific gravity (via hydrometers), refractive index
(via refractometers), and conductivity (via electronic meters).
In order to get the most out of any one of these methods,
however, aquarists must have confidence that it provides reliable
information. Each method can provide perfectly adequate information
for aquarists, assuming that the device used is properly manufactured,
calibrated, and used. Alternatively, each device can provide
misleading information if any of these factors is not optimal.

Calibration of an analytical instrument
is the best way of ensuring that the information that it is
providing is accurate. While there are many ways to calibrate
instruments for salinity determination, the simplest is to
test the instrument using a solution with a known salinity.
In that case, it is best if that standard has a salinity close
to the samples likely to be tested, which in this case is
close to natural seawater. If the instrument reads the appropriate
value, it will be suitable for use by the aquarist. If not,
then the device, or the aquarist's interpretation of the results,
might need to be altered ("calibrated") to give
the correct reading.

Aquarists can purchase commercial calibration
standards that will permit calibration of each of these methods
to a high degree of accuracy. These standards, however, can
be expensive, and in many cases, complicated to use. One reason
that commercial standards can be difficult to use, for example,
is that they usually come standardized to units of measurement
related to the technique, rather than to the unit of measurement
the aquarist needs: conductivity for conductivity meters,
refractive index for refractometers, and density or specific
gravity for hydrometers.

Unfortunately, the reef aquarium hobby
has seen a rapid expansion of devices for measuring salinity
that report in units that they are not actually measuring.
For example, how can a hobbyist use a refractive index standard
(such as a solution with a known refractive index of 1.3850)
when the refractometer reads in units of specific gravity?

This article describes a series of homemade
calibration standards that can be made from sodium chloride
(table salt) and purified freshwater. These solutions can
be used to calibrate refractometers, hydrometers, and conductivity
probes. While not likely to be as accurate as commercial standards
(depending on the ability to accurately measure weights and
volumes), they will be adequate for most reef aquarium purposes.
At the very least, they can be used to prevent a seriously
defective device from causing an aquarist to provide a grossly
inappropriate salinity level in a reef aquarium.

Table 1 shows the relevant properties of
seawater as a function of salinity. In order to make a standard
for each method, it is necessary to determine what concentration
of sodium chloride solution matches the appropriate property
of seawater.1-3 In this
article, I will use solutions with the same properties as
seawater with a salinity of 35 PSU (often written as S=35).
PSU is an acronym for practical salinity units, which is essentially
a modern replacement for ppt, since salinity is no longer
defined as directly relating to solids in the water, but rather
by its conductivity. How each standard is made and used is
detailed for each of the different methods in subsequent sections.

Table 1. Specific
gravity, conductivity, and refractive index as a function
of salinity of seawater.The darker blue rows
represent the range usually encountered in the open
ocean.

As far as I know, there is little evidence
that keeping a coral reef aquarium at anything other than
a natural salinity level is preferable. It appears to be common
practice to keep marine fish, and in many cases reef aquaria,
at somewhat lower than natural salinity levels. This practice
stems, at least in part, from the belief that fish are less
stressed at reduced salinity. Substantial misunderstandings
also arise among aquarists as to
how specific gravity really relates to salinity, especially
considering temperature effects.

Ron Shimek has discussed salinity on natural
reefs in a previous
article. His recommendation, and mine as well, is to maintain
salinity at a natural level. If the organisms in the aquarium
are from brackish environments with lower salinity, or from
the Red Sea with higher salinity, selecting something other
than S=35 may make good sense. Otherwise, I suggest targeting
a salinity of S=35 (specific gravity = 1.0264; conductivity
= 53 mS/cm).

Making Standards with Table Salt

Making salinity standards with ordinary
table salt requires the ability to make salt solutions of
known concentration. The standards given in this article are
all best made using accurate weight measurements, both of
the salt and the water. Most aquarists, however, do not have
access to high quality balances, so volume-based measurements
will be provided. Typically, they will not be as accurate
as weight-based measurements, but will be adequate for most
aquarium purposes.

A recent article in an online
culinary magazine suggested that measuring spoons used
by cooks are actually a fairly accurate way to measure volumes.
In particular, they showed that of the many teaspoons tested,
all were within 1% of the standard volume:

"Measuring spoons don't usually get a lot of consideration:
bought once and done. But have you ever wondered if your
set of spoons is accurate? Would an expensive set do a better
job? To find out, the test kitchen purchased 10 different
sets of measuring spoons, made from both plastic and stainless
steel and ranging in price from $1.99 to $14.99.

We were prepared for large differences in degree of
accuracy but found none. All of the spoons weighed in within
a few hundredths of a gram of the official standard-not
enough to compromise even the most exacting recipe."

Consequently, aquarists can use measuring
spoons for measurement of salt volumes. If they have an accurate
balance, all the better. In that case, they should just use
the mass specified, rather than the volume.

To measure with a measuring spoon or cup
that is measured to the top, first overfill it, then use the
back of a knife to carefully level the volume. I did this
with a variety of different measuring spoons and cups using
Morton's Iodized Salt, and got the following results:

Figure 1. Salt creep is one way that salinity can change
over time. This extreme case
was captured in a photo by Bob Bottini (aquababy) owner of
Tanks alot!

In this case, only the ½ cup deviated
from a fairly standard value. Overall, the Salt
Institute suggests that 1 teaspoon of a variety of different
salt brands weighs about 6 grams (1.22 g/dry mL):

"The density of granulated evaporated salt varies
depending on crystal size, structure, gradation, and degree
of compaction. The reported range of densities is 1,200-1,300
g/L." We will use 1.217 g/mL, which gives 6 grams per
teaspoon."

Measuring the water's volume is best done
with an accurate measuring container of appropriate dimensions.
In the absence of such a container, however, I have measured
a container whose volume may be standardized across the United
States, and which may therefore allow reasonably accurate
volume measurement. In particular, a plastic 2-L Diet Coke
bottle filled to the absolute top contained 2104.4 grams (mL)
of water. In a pinch, these containers may serve well as volume
standards (at least until the company changes bottle styles).

[[Notice added
post-publication: the standards described here that
use Coke bottles are subject to variation in the volume of
a 2-L Coke bottle. It has recently come to my attention that
such 2-L bottles can vary in total volume, and that this can
lead to at least a 1 ppt error in the salinity of the standards
matched to seawater salinity of 35 ppt. Standards made with
accurate measurements of salt and water should still accurately
match 35 ppt.]]

Refractometer Standard

It is widely believed that only pure water
is required to calibrate refractometers. That fact is true
of many refractometers, and is certainly appropriate for routine
calibration, but it assumes that they were manufactured correctly
and have not been damaged since manufacturing. As refractometers
used by aquarists become less and less expensive (with some
now selling for less than $30), there is every reason to believe
that at some point they will no longer be accurate enough.

The only way to be sure that a given refractometer
gives useful information is to check its accuracy in a solution
similar to aquarium water. I believe that all refractometers
should be checked in this fashion when first purchased, and
again any time there is a reason to be concerned. For example,
an aquarist might be concerned if an aquarium that had been
running for years at a salinity of 35 ppt suddenly reads 39
ppt.

In order to provide a standard for refractometers,
a solution whose refractive index is similar to normal seawater
is required. Seawater with S= 35 has a refractive index of
1.3394.1 Likewise, the refractive
index of different sodium chloride solutions can be found
in the scientific literature. My CRC Handbook of Chemistry
and Physics (57th Edition,
Page D-252)4 has such a
table. That table has entries for 3.6 and 3.7 weight percent
solutions of sodium chloride that span the value for normal
seawater. Interpolating between these data points suggests
that a solution of 3.65 weight percent sodium chloride
has the same refractive index as S=35 seawater, and can be
used as an appropriate standard (Table 2).

Table 2. Refractive
Index as a function of the concentration of a sodium
chloride solution.1,4The darker blue row represents the standard.

Sodium
Chloride Concentration (weight %)

Refractive
Index

Salinity
(PSU)

3.3

1.3388

31.65

3.4

1.3390

32.8

3.5

1.3391

33.3

3.6

1.3393

34.4

3.65

1.3394

35.0

3.7

1.3395

35.6

3.8

1.3397

36.7

This 3.65 weight percent sodium chloride solution can be made by dissolving 3.65 grams of sodium chloride in 96.35 grams (mL) of purified freshwater.

For a rougher measurement in the absence
of an accurate water volume or weight measurement:

1. Measure ¼ cup of Morton's Iodized
Salt (about 73.1 g)
2. Add 1 teaspoon of salt (making about 79.3 g total salt)
3. Measure the full volume of a plastic 2-L Coke or Diet
Coke bottle filled with purified freshwater (about 2104.4
g)
4. Dissolve the total salt (79.3 g) in the total water volume
(2104 g) to make an approximately 3.65 weight percent solution
of NaCl. The volume of this solution will be slightly larger
than the Coke bottle, so dissolve it in another container.

[[Notice added
post-publication: the standards described here that
use Coke bottles are subject to variation in the volume of
a 2-L Coke bottle. It has recently come to my attention that
such 2-L bottles can vary in total volume, and that this can
lead to at least a 1 ppt error in the salinity of the standards
matched to seawater salinity of 35 ppt. Standards made with
accurate measurements of salt and water should still accurately
match 35 ppt.]]

How to Use a Refractive Index Standard

One simple way to use this refractive index
standard is to measure it with a refractometer, and just remember
what setting the standard came to. That setting represents
S=35 seawater, with all of the properties shown in Table 1.
Hopefully, the reading of the refractometer at that point
will be similar to the properties in Table 1 (specific gravity
= 1.026 - 1.027, or S=35, depending on the units). Simply
using it as the target salinity for the aquarium is a fine
way to go.

Alternatively, one can actually calibrate
the refractometer using the standard by adjusting it until
it reads the appropriate setting indicated in Table 1. Exactly
how to adjust it depends on the refractometer, but often it
is as simple as turning a screw.

Specific Gravity Standard

Most aquarists recognize that inexpensive
hydrometers are often prone to error. In some cases, inaccuracy
is due to poor manufacturing, and in other cases it is due
to poor usage by aquarists. In a previous
article I tested several hydrometers and found variable
results, from good to marginal. Beyond the inherent accuracy
of the measurement is the confusing problem of how specific
gravity relates to the temperature of the measurement, an
issue which I detailed in that same article.

The best way to be sure that a given hydrometer
is giving accurate information is to check its accuracy in
a solution with a density (specific gravity) similar to the
aquarium water. In order to provide a standard for hydrometers,
a solution of a similar specific gravity to normal seawater
is required. Seawater with S= 35 has a specific gravity of
about 1.0264 (Tables 1 and 3).

Table 3. Density
and specific gravity as a function of salinity of seawater.3The darker blue rows represent the range usually
encountered in the open ocean.

Salinity
(PSU)

Density
(25° C)

Specific
Gravity (25° C)

0

997.05

1.0000

29

1018.8

1.0218

30

1019.6

1.0226

31

1020.3

1.0233

32

1021.1

1.0241

33

1021.8

1.0249

34

1022.6

1.0256

35

1023.3

1.0264

36

1024.1

1.0271

37

1024.9

1.0279

38

1025.6

1.0286

39

1026.4

1.0294

In order to match this specific gravity
to a standard solution made from sodium chloride, look up
the density of different sodium chloride solutions in the
scientific literature. My CRC Handbook of Chemistry and Physics
(57th Edition, Page D-252)4
has such a table (partially reproduced in Table 4), but it
has data only for 20°C (68°F). Specific gravity
at 20°C is then easily calculated by dividing the density
of the solutions by the density of water at the same temperature.
This table (4) can then be compared to seawater at 20°C
(Table 5). The primary purpose of showing specific gravity
at 25°C (77°F; Tables 1 and 3) and 20°C (Table
4) is to show that specific gravity does not change much with
temperature (1.0264 vs. 1.0266). Nevertheless, it is only
the 20°C data that will be used to devise a standard.

The table in the CRC Handbook has entries
for 3.7 and 3.8 weight percent solutions of sodium chloride
that span the specific gravity value for normal seawater.
Interpolating between these data points suggests that a
solution of 3.714 weight percent sodium chloride has the same
specific gravity (and density) as S=35 seawater, and can be
used as an appropriate specific gravity standard (Table
5). For most purposes, 3.7 weight percent is accurate enough.

Table 4. Density
and specific gravity as a function of salinity of seawater
at 20° C.4The darkened blue rows represent the range usually
encountered in the open ocean.

Salinity
(PSU)

Density
(25° C)

Specific
Gravity (25° C)

0

988.2

1.0000

29

1020.2

1.0220

30

1021.0

1.0228

31

1021.7

1.0236

32

1022.5

1.0243

33

1023.2

1.0251

34

1024.0

1.0258

35

1024.8

1.02660

36

1025.5

1.0274

37

1026.3

1.0281

38

1027.1

1.0289

39

1027.8

1.0297

Table 5. Specific
gravity as a function of the concentration of sodium
chloride in water. The values in the medium blue boxes
are interpolated and the darker blue row represents
the standard.3,4

Sodium
Chloride Concentration (weight %)

Specific
Gravity
at 20° C

Salinity

3.4

1.0243

32.0

3.5

1.0250

32.9

3.6

1.0257

33.8

3.7

1.0265

34.8

3.71

1.02657

34.9

3.714

1.02660

35.0

3.72

1.02664

35.1

3.73

1.02671

35.1

3.74

1.02678

35.2

3.8

1.0272

35.8

To produce a 3.714 weight percent sodium
chloride solution, dissolve 1 teaspoon (6.20 grams) of Morton's
Iodized Salt in 161 mL (161 g) of freshwater (making a total
volume of about 163 mL after dissolution of the salt). This
solution can be scaled up as desired.

For a rougher measurement in the absence
of an accurate water volume measurement:

1. Measure ¼ cup of Morton's Iodized
Salt (about 73.1 g)
2. Add 1½ teaspoon of salt (making about 82.4 g total
salt)
3. Measure the full volume of a plastic 2-L Coke or Diet
Coke bottle filled with purified freshwater (about 2104.4
g)
4. Add an additional 2 tablespoons of purified freshwater
(about 30 g)
5. Dissolve the total salt (82.4 g) in the total water volume
(2134.4 g) to make an approximately 3.7 weight percent solution
of NaCl. The volume of this solution is larger than the
Coke bottle, so dissolve it in another container.

[[Notice added
post-publication: the standards described here that
use Coke bottles are subject to variation in the volume of
a 2-L Coke bottle. It has recently come to my attention that
such 2-L bottles can vary in total volume, and that this can
lead to at least a 1 ppt error in the salinity of the standards
matched to seawater salinity of 35 ppt. Standards made with
accurate measurements of salt and water should still accurately
match 35 ppt.]]

How to Use a Specific Gravity Standard

Depending on the type of hydrometer, one
would use this solution differently.

For standard floating hydrometers (Figure
2), which are not
self-correcting for temperature variations, it is important
to use the standard at the same temperature at which the aquarium
water will be tested (within say, ± 0.5 °C or
± 1 °F). Preferably, that will also be the temperature
at which the hydrometer is intended to be used (often marked
on it), but that is not an absolute requirement. The aquarist
can then mark on the hydrometer the level to which it rises
(that is, the water line), and use that as an indication of
the specific gravity of S=35 seawater, which has all of the
properties listed in Table 1(specific gravity = 1.0264, etc).
If the hydrometer reads higher or lower than 1.0264, then
the aquarist can just imagine the scale on the hydrometer
to be shifted up or down, and shift all other
readings taken with it (at the same temperature) by the same
amount.

Figure 2. The Tropic Marin floating hydrometer.

For example, if the standard comes out
at 1.0230 (and it is really 1.0264), then just add 1.0264
- 1.0230 = 0.0034 to each measured value).

For swing arm hydrometers (Figure 3), which
are largely self-correcting
for temperature variations, add the standard to the swing
arm hydrometer at roughly the same temperature at which the
aquarium water will be tested (say, ± 5°C or ±
10°F). Once the reading stabilizes, the aquarist can
mark the reading (or just remember it) and use that as an
indication of the specific gravity of S=35 seawater, which
has all of the properties listed in Table 1 (specific gravity
= 1.0264, etc). If the hydrometer reads higher or lower than
1.0264, then the aquarist can just imagine the scale on the
hydrometer to be shifted up or down, and shift all other readings
taken with it by the same amount, just as for a standard floating
hydrometer.

Figure 3. The SeaTest swing arm hydrometer.

Just to be especially clear: this solution
need not be used at exactly 20°C (68°F). It will
be just about as accurate at 25°C (77°F) since specific
gravity does not change much with temperature, and these salt
solutions would be expected to change density with temperature
in about the same fashion as seawater. The most important
factor is that the temperature of the standard, when measured,
be the same as the aquarium water when it is measured..

How to Use a Standard Hydrometer

Here are a few additional tips for using
a hydrometer:

1. Make sure that the hydrometer is completely
clean (no salt deposits) and that the part of the hydrometer
above the water line is dry. Tossing it in so it sinks deeply
and then bobs to the surface will leave water on the exposed
part that will weigh down the hydrometer and give a falsely
low specific gravity reading. Salt deposits above the water
line will have the same effect. If any deposits won't easily
dissolve, try washing it in dilute acid (such as vinegar
or diluted muriatic acid).
2. Make sure that there are no air bubbles attached to the
hydrometer. These will help buoy the hydrometer and yield
a falsely high specific gravity reading.
3. Make sure that the hydrometer is the same temperature
as the water (and preferably the air).
4. Read the hydrometer at the plane of the water's surface,
not along the meniscus (Figure 2; the meniscus is the lip
of water that either rises up along the shaft of the hydrometer,
or curves down into the water, depending on the hydrophobicity
of the hydrometer).
5. Rinse with purified freshwater after use to reduce deposits.
6. Do not leave the hydrometer floating around in the tank
between uses. If left in the aquarium, deposits may form
that will be difficult to remove.

How to Use a Swing Arm Hydrometer

In addition to those described above, here
are some special tips for swing arm hydrometers:

7. Make sure that the hydrometer is completely
level. A slight tilt to either side will change the reading.
8. Some swing arm hydrometers recommend "seasoning"
the needle by filling it with water for 24 hours prior to
use. This presumably permits the water absorbed into the
plastic to reach equilibrium. In the case of the hydrometer
that I tested in a previous article, the hydrometer became
slightly less accurate after "seasoning."

Conductivity Standard

Conductivity can readily be used to measure
the salinity of seawater. In a previous
article I detailed how this measurement works and why
it is suitable for reef aquaria. In short, the more ions there
are in solution, the more easily the solution will conduct
electricity. In fact, conductivity is so easily measured and
standardized that it forms the basis of the modern definition
of salinity, PSU (Practical Salinity Units). S=35 seawater
is defined as seawater with the same conductivity as a solution
made from 3.24356 weight percent potassium chloride (KCl),
and that conductivity is exactly 53 mS/cm (mS/cm is one of
the units used for conductivity, it is milliSiemens per centimeter).
Higher and lower conductivities give higher and lower salinities,
respectively, using a complicated equation that will not be
discussed here.

There are two ways to formulate a conductivity
standard that matches S=35 seawater. The first is looking
to the scientific literature to see what sodium chloride solutions
provide a conductivity of 53 mS/cm. A second is to match a
sodium chloride solution to the conductivity of 3.24 weight
percent potassium chloride in water. This article does both.

First, the scientific literature. Fortunately,
many
measurements of conductivity of such solutions have been
made over the years. Without going into detail about how they
were measured, the data from these papers indicate that a
53 mS/cm conductivity solution is provided by a 33.64 g/L
(0.576 M) sodium chloride solution. That solution corresponds
to 3.29 weight percent sodium chloride.2

Alternatively, one can measure conductivity
of salt solutions. I made a solution of 3.24 weight percent
KCl in deionized water and measured its conductivity. The
reading on the uncalibrated meter was 52.5 mS/cm (it would
have been 53 mS/cm with a perfectly calibrated meter).

I then made a solution of deionized water
and Morton's Iodized Salt, adding salt until I matched the
conductivity of the prior solution. It required EXACTLY 3.29
weight percent sodium chloride to match this conductivity.
Believe it or not, I didn't even recognize the close agreement
between these two methods during the test, as I hadn't worked
through the math until long after taking the original measurements.

So not only is there good evidence that
a 3.29 weight percent sodium chloride solution is appropriate,
but additional evidence demonstrates that Morton's Iodized
Salt from a grocery store is a suitable material for this
purpose.

To make a 3.29 weight percent sodium chloride
solution, dissolve 1 teaspoon (6.20 grams) of Morton's Iodized
Salt in 182 mL (182 g) of freshwater (making a total volume
of about 184 mL after dissolution of the salt). This solution
can be scaled up as desired.

For a rougher measurement in the absence
of an accurate water volume measurement:

1. Measure ¼ cup of Morton's Iodized
Salt (about 73.1 g)
2. Measure the full volume of a plastic 2-L Coke or Diet
Coke bottle filled with purified freshwater (about 2104.4
g)
4. Add 3 tablespoons of purified freshwater (about 45 g)
5. Dissolve the total salt (73.1 g) in the total water volume
(2149.4 g) to make an approximately 3.29 weight percent
solution of NaCl. The volume of this solution is larger
than the Coke bottle, so dissolve it in another container.

[[Notice added
post-publication: the standards described here that
use Coke bottles are subject to variation in the volume of
a 2-L Coke bottle. It has recently come to my attention that
such 2-L bottles can vary in total volume, and that this can
lead to at least a 1 ppt error in the salinity of the standards
matched to seawater salinity of 35 ppt. Standards made with
accurate measurements of salt and water should still accurately
match 35 ppt.]]

How to Use a Conductivity Standard

How to best use a conductivity standard
depends a bit on the meter involved. If the meter can be calibrated,
then my suggestion is to get the solution to about 25°C
(exactly that temperature if the meter doesn't automatically
compensate for temperature, but that would be unusual) and
then adjust the meter until it reads 53 mS/cm or S=35 (depending
on the output).

Many meters, however, do not allow such
calibration. In that case, measure the conductivity or salinity
of the standard, and then set up a correction ratio that is
applied manually. For example, if the standard reads 56 mS/cm,
then multiply all readings on that meter by 53/56 (0.946)
to get a corrected reading. The same correction could apply
to salinity. For example, if it reads S=38 (or 38 ppt), then
multiply every reading by 35/38 = 0.921.

Alternatively, the simplest way is to use
the value that is found from the standard as the target for
the aquarium, and not worry about calibrations or corrections.

Summary

This article provides a way for reef aquarists
to make and use salinity standards for the most common ways
of measuring salinity: refractometers, hydrometers, and conductivity
meters. Hopefully, these will help aquarists avoid problems
that might arise from poorly calibrated devices, or at least
ease their concerns about whether or not their devices are
working properly.

Happy Reefing!

If you have any questions about this article, please visit my author
forum on Reef Central.

References:

1. The data on the refractive index of
seawater as a function of salinity were obtained from: